1. Field of the invention
The present invention relates to a tube combination, more particularly a tube combination, which has a coupling for connecting two tube parts thereof together, which is relatively easy and safe to use and relatively economical to manufacture.
2. Brief Description of the Prior Art
Steel tubes are widely used on furniture such as beds, desks, racks, and chairs because they are light in weight and can be easily shaped. Early conventional steel tubes were not formed with ornamental designs on the cylindrical surfaces therefore they can't match other parts of furniture. To make steel tubes look attractive and match other parts of furniture, annular trenches are formed on the steel tubes. The annular trenches can also help other parts of furniture located when they are joined to the steel tubes.
Referring to FIGS. 7, 8, and 9, a tube combination has a coupling 42, upper and lower tubes 41, 41, which have spaced annular trenches 411 on the outer sides and threads on inner sides of two ends, and are joined together by means of the coupling 42; the coupling 42 has threads 421, 421 on outer sides of upper and lower ends, and a middle rim 422, which is made with such a shape as to make the joint between the tubes 41, 41 look like the trenches 411 when the coupling 42 are connected to the tubes. And, when the tube combination is joined to grilles 43 to form a rack, as shown in FIGS. 8 and 9, connecting tube elements 432 of the grilles 43, which have annular protrusions 433 on inner sides, can engage selected ones of the annular trenches on the tube combination easily. In other words, the grilles 43 can be easily located and secured in position owing to the annular trenches. However, the tube combination is found to have disadvantages as followings:    1. The coupling 42 is costly to manufacture because it is formed with threads 421 on two ends, and a middle rim 422, which is bigger than the threaded ends in diameter, and in turn, the coupling 42 has to be made by means of cutting deeply into the curved surface of a cylindrical element with a lathe, cutting a lot of portions out of the cylindrical element. Furthermore, the coupling 42 has to be made of a material of solid cylindrical shape instead of a hollow cylindrical tube because the material has to have relatively big thickness for allowing the cutting as described above and because ordinary easily available tubes don't have such a big thickness. And, the cylindrical material has to be formed with a hole 423. Consequently, a lot of portions are cut out of the material, and become waste.    2. Manufacturing of the tube combination is difficult and labor-costing; a cylindrical material has to be formed with threads at two ends to become the coupling 42, and both of the tubes 41 have to be formed with threads on the inner sides of the ends therefore relatively many cutting processes have to be performed, and in turn, manufacturing of the tube combination is costly.    3. Assembly of the tube combination may cause injure to the persons performing the assembly; the users have to first hold a first threaded end of the coupling 42 to screw the other end into a tube 41, and then screw the other end into the other tube therefore the users are prone to injure the fingers due to the sharp ends of the threads. Furthermore, the assembly is labor-costing because the coupling 42 has to be screwed into the tubes at both ends.